I think dropping Sonnanstine for Bonderman or a rookie would be a poor decision. He's nothing special but he's healthy, proven, and reliable. Sonnanstine is a much better bet to have a "good" season than any of the guys on the wire right now.
As far as the trade itself... If you need a SS, I would do it. If you can survive without Hardy I wouldn't do it. Cueto should be pretty good this year and I disagree that his production can be replaced through free agency in a league with pitching that deep. Also, it's probably a matter of when, not if, Ray closes. So yeah, unless you NEED Hardy, I would stay put.
If that can get it done it would be better for you IMO. Either way I'd make the deal. Ray is a surer bet to close at some point this season than Corpas. With your great pitching depth Cueto is expendable and Hardy is a nice upgrade over Tejada. Good deal for you.
Hal·la·day, n. 1. every fifth day in Philadelphia. 2. a day of rest for the bullpen. 3. innings eater. 4. doc. 5. ace.